A VCO is a special type of oscillator that changes its frequency depending on a control signal, usually a voltage. A VCO has what's called a free-running frequency which is the frequency that the oscillator runs at when the control voltage is at midrange, normal operating condition. If the control voltage rises above midrange, the VCO increases the frequency of the output clock, and if the control voltage falls below midrange, then the VCO lowers the frequency of the output clock. See the definition of a Phase-Locked Loop.